Abstract
The increasing accuracy and growing time span of Very Long Baseline Interferometry (VLBI) observations allow the determination of seasonal signals in station positions which still remain unmodelled in conventional analysis approaches. In this study we focus on the impact of the neglected seasonal signals in the station displacement on the celestial reference frame and Earth orientation parameters. We estimate empirical harmonic models for selected stations within a global solution of all suitable VLBI sessions and create mean annual models by stacking yearly time series of station positions which are then entered a priori in the analysis of VLBI observations. Our results reveal that there is no systematic propagation of the seasonal signal into the orientation of celestial reference frame but position changes occur for radio sources observed non-evenly over the year. On the other hand, the omitted seasonal harmonic signal in horizontal station coordinates propagates directly into the Earth rotation parameters causing differences of several tens of microarcseconds.
Highlights
Realizations of Terrestrial Reference Frames (TRF), such as the International Terrestrial Reference Frame 2008 (ITRF 2008, Altamimi et al 2011) and the Very Long Baseline Interferometry (VLBI) Terrestrial Reference Frame 2008 (VTRF 2008, Böckmann et al 2010), define station positions as the sum of the coordinates at a certain time epoch plus a linear velocity term times the time span elapsed since the reference epoch
In this paper we investigate the propagation of the seasonal signals in station coordinates into the Celestial Reference Frame (CRF) and Earth Orientation Parameters (EOP) estimated from 24-h VLBI sessions
We compare them to two sets of hydrology loading corrections and surface displacements derived from Gravity Recovery and Climate Experiment (GRACE) and find good agreement for inland sites frequently observed by VLBI
Summary
Realizations of Terrestrial Reference Frames (TRF), such as the International Terrestrial Reference Frame 2008 (ITRF 2008, Altamimi et al 2011) and the Very Long Baseline Interferometry (VLBI) Terrestrial Reference Frame 2008 (VTRF 2008, Böckmann et al 2010), define station positions as the sum of the coordinates at a certain time epoch plus a linear velocity term times the time span elapsed since the reference epoch. Seasonal signals with amplitudes of several millimetres are still present in most of the station position time series, as shown by several authors for VLBI and the Global Positioning System (GPS) position time series, cf Titov and Yakovleva (2000), Blewitt et al (2001), van Dam et al (2001, 2012), Dong et al (2002), Petrov and Ma (2003), Ding et al (2005), Collilieux et al (2007), Tesmer et al (2009, 2011) or Eriksson and MacMillan (2014) They conclude that parts of the remaining seasonal signal have a geophysical origin, mainly from hydrology and—to a lesser extent—from nontidal ocean loading.
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